Modeling scaling of silica from reinjection waters at well head conditions in the berlin geothermal field, el salvador, central America

Conference Proceedings: Modeling scaling of silica from reinjection waters at well head conditions in the berlin geothermal field, el salvador, central America

Abstract

Generation of electricity at the Berlin Geothermal
Field (Eastern El Salvador) started in February 1992.
All the residual waters (~ 350 kg/s) produced during
the conversion of thermal to electrical energy have
been reinjected to the same field. Reinjection was the
only feasible option for this field. However, silica
scaling problems in the reinjection wells decrease the
capacity of these wells to absorb water. The
temperature of the reinjected waters is 175 oC and the
reservoir temperature at the reinjection wells ranges
from 280 ºC to 200 ºC. The mineral that presents a
higher saturation index and it is more likely to
precipitate at the pressure and temperature conditions
of the water transport pipes and wellhead is
amorphous silica. However, high precipitation of
amorphous silica is not happening in the water
transport system and the waters are reinjected
supersaturated. Modeling of cooling of the hot
reservoir waters in equilibrium with quartz and
mixing with reinjection waters was done using the
programs SOLVEQ and CHILLER.
The modeling results show that the mass of quartz
precipitated within the reservoir per unit mass of
reinjected water strongly depends on the initial silica
concentration of the reinjected water. The
temperature and mixing fraction of the reservoir does
not seem to have a deep effect if the reservoir is at a
higher temperature. Most of the silica precipitated is
coming from the reinjected water with only a minor
fraction coming from the temperature reequilibration
of the reservoir. However, a slightly higher mass
fraction of silica is precipitated when the temperature
difference between reinjected and reservoir water is
greater. Considering the flow rate of reinjected water
at well TR-1A, the volume of quartz precipitated per
kg of reinjected water, a 10% porosity, and a
thickness of the reservoir of 200 m, the volume of
pores clogged per year was found as well as the
percentage of clogged reservoir volume assuming
different radius for the deposition of quartz around
the well. A radius between 5 and 15 m for the
precipitation of quartz could explain the decline in
absorption capacity. Higher radius would give only a
few percent change in porosity that cannot explain
the change in absorption capacity. As the clogging of
the pores seems to happen close to the wells,
hydraulic effects are proposed to explain the abrupt
response of nearby wells when reinjection starts in a